There are a variety of mechanical seal designs available for nuclear primary coolant pumps. The function of these mechanical seals is to restrict the leakage of hot, high pressure reactor coolant system (RCS) water from the reactor primary system into the reactor containment vessel, while allowing a rotating shaft to penetrate the primary pressure boundary. The shaft drives a pump impeller, and the mechanical seal is located along the shaft in a seal chamber. These pumping systems require cooling to the mechanical seal in order to provide the kind of operating environment for the seal that will enable optimum performance. Conditions in these pumping systems may be as high as 2500 psi and 650 F, and it is necessary to ensure adequate life for the seal under these conditions by the provision of a cooling system that cools the mechanical seal.
Under emergency conditions where electrical power or control may be lost to the cooling system, seal cooling may be lost and excessive high temperatures at the seal faces would occur. This high temperature may cause the seals to be compromised due to a variety of reasons leading to a possibility of greatly increased RCS leakage to the reactor containment vessel.
It is an object of the invention, therefore, to provide a safety backup seal (abeyance seal) that actuates under specified conditions of leakage, such as during loss of the cooling system, and thereby maintains a leak tight seal against full RCS conditions at the seal for the duration of the emergency.
One other design concept is disclosed in a Westinghouse patent (Application US 2010/0150715 A1). This patent discloses a thermally actuated backup seal for a nuclear power plant that requires an elevated temperature in the range of 250 to 290 F to melt a spacer or wax filled piston, wherein melting of such structure then allows a retaining pin to retract allowing a split piston ring to collapse against the shaft. Further pressure and temperature increases result in a secondary polymer ring to also engage the shaft to provide even lower leakage than what would be provided by the first metal split ring, which might leak due to the gap of the split. The solid polymer ring is located downstream from the split ring.
This design, however, embodies various disadvantages. For example, it may take up to 45 seconds after the temperature of activation is reached (250-290 F) to actuate, wherein significant amounts of steam could escape before actuation occurs.
Further, this design can be inadvertently actuated by momentary loss of cooling in non-accident (including hot standby) conditions, and it may be difficult or impossible to determine if it is actuated, and if inadvertent actuation is not discovered, the seal may not be available for a subsequent true emergency.
Also, the seal can be actuated under rotation by any deficiencies of its companion primary seal, and if the seal actuates during shaft rotation, the seal will damage itself and/or the rotating components due to the rubbing and possible seizing of the metal piston ring, which may result in leakage far above the leakage rates identified in testing.
Still further, the seal provides no protection to gross liquid leakage and full actuation does not provide zero-leakage. At 2250 psi and 575 F the leakage is reported in the patent application to be on the order of 0.1 gal/min. If extended accident scenarios up to 168 hours occur, erosion of the sealing ring may significantly increase leakage beyond these values.
Additionally, the downstream polymer element has to be able to seal a 0.050″ radial gap between the housing flange and shaft. It is well understood that at temperatures above 600 F the element material has a potential to flow which would result in a loss of sealing capability due to extrusion.
More particularly as to the invention, the invention is believed to overcome the disadvantages associated with prior seal designs. The invention is an abeyance seal which comprises the following:
Metal Thermal Expansion Preventer (TEP): The TEP mounts with a seal housing radially adjacent to the shaft and has an interference fit to a metal sealing/anti-extrusion ring to facilitate a unitized assembly. The TEP provides a limiting restriction on the horizontal (axial sealing portion) lip of the polymer actuation ring. Since the coefficient of thermal expansion for the polymer actuation ring is much greater than for metal components, the TEP prevents the polymer material from growing away from the shaft or shaft sleeve, thereby maintaining a constant and controlled gap between the rotating and stationary components under elevated temperature conditions. Also as part of this ring are a number of openings in the front face (high pressure side) located at or below the centroid of the polymer actuation ring and a tapered front edge where it also mates with the polymer actuation ring. These two features facilitate the actuation process under high vapor and or two-phase flow velocities due to mass momentum.
Polymer Actuation Ring: This device is the first line of sealing actuation. Because it is made of a flexible polymer compound, when subjected to leakage this element will rotate about its centroid, collapsing against the shaft or shaft sleeve and forming the initial sealing function between the rotating and stationary components (see FIG. 2). This action is the result of the application of the Bernoulli Effect where a small differential pressure is caused by increased leakage through the gap between the horizontal portion of the polymer ring and the shaft or shaft sleeve. This is further aided by the aforementioned fluid impingement due to momentum of leakage flow directed through the openings in the TEP ring below the centroid of the polymer ring. Once the sealing lip begins to contract towards the shaft or shaft sleeve, the differential pressure is further exaggerated resulting in the lip accelerating in its closing action. Once the gap starts to close between the lip and shaft or shaft sleeve, full actuation has been shown, through testing, to occur in less than one second. Once the polymer ring seals to the shaft or shaft sleeve the differential pressure across the abeyance seal begins to rise rapidly. FIG. 3 shows the condition of the polymer ring at 100 psi.
The polymer ring further seals at the shaft or shaft sleeve and also at its outside diameter against the metal sealing/anti-extrusion ring and thermal expansion preventer (TEP) ring. This deformation of the polymer ring is further facilitated by the fact that the outside diameter of the ring is unconstrained. If the OD were constrained the ring becomes stiffer raising the internal stress at the constrained region and reducing the actuation capability. Actuation of this element will occur at significantly lower leakage flows due to high velocity steam, two phase flow, or gas. Much higher flows of liquid water will pass through before actuation occurs.
Metal Sealing/Anti-Extrusion Ring: The metal sealing/anti-extrusion ring has an interference fit to the metal backing ring. This interference fit is performed before the polymer actuation ring and TEP ring are installed. This again is done to provide a unitized assembly, as well as to seal the metal sealing/anti-extrusion ring to the metal backing ring. From a function standpoint, the polymer ring is to provide the initial sealing function due to leakage at low differential pressure (e.g. less than 10 psid). Once the polymer ring seals to the shaft or shaft sleeve, increased sealed pressure that would result loads the polymer ring against the metal sealing/anti-extrusion ring causing it to deform, rotating so that the inside diameter of the metal ring comes into contact with the shaft or shaft sleeve. The metal ring will come into full contact with the shaft or shaft sleeve at pressures less than 1000 psi (see FIG. 4). The anti-extrusion ring now forms an additional sealing function and also prohibits the polymer ring from extruding at higher pressures and temperatures. Because the polymer ring cannot extrude, it now is able to assist in further leak tight sealing under higher pressure and temperatures. FIG. 5 shows the complete sealing of this arrangement at 2500 psi and 500 F. The metal sealing/anti-extrusion ring is constructed as a separate component to facilitate manufacture of the thin section. If alternatively the metal sealing/anti-extrusion ring and the metal backing ring were formed as one piece, the function would be the same.
Metal Backing Ring: Provides a support component to the Metal sealing/anti-extrusion ring.